[services] Tunnelbroker - Add device ping interval to Constants

Summary:
This diff introduces a ping interval to check if the device is online.
The default "beacon" value for pinging is 3000 ms in our keyserver, which seems reasonable.
More context can be found in the Linear task.

Related Linear task: ENG-1766

Test Plan: No test, just a constant value.

Reviewers: jon, varun, tomek, karol, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, tomek, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D5151